Cancer is a formidable cell proliferative disease that takes millions of lives each year. With breast cancer alone, more than 50,000 women in the United States are afflicted annually. Breast tumors are traditionally detected by ultrasound, MRI, or mammography followed by histology, and are treated by surgical resection followed by chemo- and/or radiation therapy. Mortality from breast (and other) cancers can be reduced through early diagnosis and treatment of tumors (Kelsey J L, Epidemiol. Rev. 1: 74-109, 1989). An effective method for the early detection of tumors is scintigriphic imaging with tumor-specific radioactive imaging agents.
A number of radioactive tumor imaging agents have been used for detecting breast tumors, with varying degrees of success. For example, receptor specific biomolecules, such as neuropeptides, bind to receptors in nanomolar concentrations and have been the focus of a considerable interest both in the therapeutic and diagnostic fields (Fischman A J et al., J. Nucl. Med. 34: 2253-2263, 1993; Hokfelt T Neuron 7: 867-879, 1991).
However, the only commercially available neuropeptide imaging agent, 111In-[DTPA-D-Phe1] Octreotide, has not been highly successful in detecting breast tumors (van Eijck C H J et al. Lancet 343: 640-643, 1994; McCready V R et al. Lancet 343: 617, 1994). van Eijck et al., 1994, supra report that in 52 primary breast cancers, only 75% positive scintigraphy was achieved with this agent. Furthermore, van Eijck et al. showed that imaging of axillae with 111In-[DTPA-D-Phe1] Octreotide detected non-palpable, cancer-containing lymph nodes in only 4 of 13 patients with histologically-proven metastases. The low efficacy of 111In-[DTPA-D-Phe1] Octreotide for imaging breast tumors is attributed to the low density of oncogene receptors expressed on breast tumor cells which are specific for the agent. The usefulness of 111In-[DTPA-D-Phe1] Octreotide for imaging breast tumors is therefore limited.
123I-Tyr3-octreotide has also been used for radio-diagnostic imaging, but this agent has not been evaluated for imaging of breast tumors (Krenning E P et al., Eur. J. Nucl. Med. 20: 716-731, 1993). However, based on the poor ability of 111In-[DTPA-D-Phe1] Octreotide to image breast tumors, 123I-Tyr3-octreotide is not expected to be a useful breast tumor imaging agent.
In any case, radio-iodinated agents are generally not desirable for use as imaging or therapeutic agents, because approved radio-iodinated radiopharmaceuticals normally cannot be prepared at the clinical site. Radio-iodinated agents have specific limitations as well; for example, 125I-labeled agents are typically not used for imaging applications due to the relatively long half-life (about 59 days) and low emission energy of the radionuclide. 123I-labeled agents are not preferred, because 123I is a cyclotron-generated radionuclide which is expensive to produce and the radionuclide has too short a half-life (13.3 hours) to be commercially useful. 131I-labeled agents have too high an emission energy for quality scintigriphic imaging.
Technetium-99m (99mTc) is widely used in diagnostic imaging agents because it emits gamma radiation at 140 KeV, has a physical half-life of 6 hours, and is easily produced on-site using a molybdenum-99/99mTc generator. The shorter half-life of 99mTc minimizes the radiation dose to normal organs, and its emission energy allows efficient detection by gamma cameras. 99mTc is therefore the radionuclide of choice in nearly 90% of clinical nuclear medicine applications.
Imaging agents are typically labeled with 99mTc through a metal chelating moiety. The 99mTc metal chelating moiety is generally also able to complex therapeutic radionuclides such as 186Re and 188Re. Thus, a single agent comprising a metal chelator can advantageously be used as a diagnostic or therapeutic agent, depending on which radionuclide is employed.
U.S. Pat. No. 6,395,255 to M. Thakur discloses a method for labeling a vasoactive intestinal peptide (VIP)-based agent using a 99mTc chelator. The chelator and labeling chemistry disclosed in U.S. Pat. No. 6,395,255 is suitable for labeling VIP agents with either 99mTc or rhenium radionuclides. However, the VIP agents disclosed in U.S. Pat. No. 6,395,255 bind only to tumor cells expressing VIP receptors. As certain tumors express other types of receptors at high density, an imaging or therapeutic agent which can bind to a wider range of receptors expressed by these tumor cells would be advantageous.
Pituitary adenylate cyclase activating peptide (PACAP) is a 38-amino acid peptide originally isolated from bovine hypothalamus (Miyata A et al, Biochem. Biophys. Res. Commun. 164: 567-574, 1989). This peptide stimulates the accumulation of intracellular and extracellular cAMP in monolayer cultures of rat anterior pituitary cells (Gottschall P E et al., Endocrinology 127: 272-277, 1990). Gottschall et al., 1990, supra isolated a 27-amino acid PACAP (PACAP27) from bovine hypothalamus, and concluded that PACAP38 and PACAP27 were equally active, and were derived from a single 176-amino acid precursor.
PACAP27 is ten times more potent than the 28-amino acid vasoactive intestinal peptide VIP28 in stimulating adenylate cyclase in pituitary cells (Gottschall et al., 1990, supra). 125I-PACAP27 is also capable of displacing VIP28 bound to normal lung membrane. The IC50 value for VIP28 is approximately 15 nM, and the IC50 value for PACAP27 is approximately 1.5 nM.
PACAP27 binds with high affinity to PACAP, VIP-R1 and VIP-R2 receptors, whereas VIP28 binds with high affinity only to VIP-R1 and VIP-R2 receptors (Zia F et al., Cancer. Res. 55: 4886-4891, 1995). The PACAP, VIP-R1 and VIP-R2 receptors, referred to collectively as VPAC receptors (Reubi J C, J. Nucl. Med. 36: 1846-1853, 1995; Reubi J C et al., Cancer Res. 60: 1305-1312, 2000), are expressed in high density on breast tumor (Zia H et al., Cancer Res. 56: 3486-3489, 1996) and other tumor cells (Harmar T et al., TiPs 15: 97-98, 10 1994; Reubi J C et al., Eur. J. Nucl. Med. 24: 1058, 1997; Le Meuth V et al., Amer. J. Physiol. 260: G265-74, 1991; Basille M et al. Brain Res. 82: 1-2, 1994; Vertrongen P et al., Neuropeptides 30: 491-496, 1996; Olianas M C et al., J. Neurochem. 67: 1292-1300, 1996; Lelievre V et al., Neuropeptides 30: 313-322, 1996; and Parkman H P et al., Regulatory Peptides 71: 185-190, 1998). For example, tumors (other than breast tumors) which express VPAC receptors include ovarian, endometrial, prostate, bladder, lung, esophageal, colonic, pancreatic, neuroendocrine and brain tumors.
However, there has been little success in producing a targeted radiopharmaceutical using PACAP. See, for example, Reubi J C et al., Eur. J. Nucl. Med. 24: 1058, 1997, who showed that the biological activity of PACAP27 linked to DTPA at its N-terminus was reduced from 100 to <0.01.
What is needed, therefore, is a radioactive tumor imaging or therapeutic agent based on a receptor-specific biomolecule such as PACAP, which binds with high affinity to one or more types of receptors present on certain tumor cells. The imaging or therapeutic agent should ideally be labeled with a diagnostic or therapeutic radionuclide through a metal chelator.